Portion of a chair back



FIG. 1 is a right side elevational view of a portion of a chair back according to my new design, the left side being a mirror image of the right side;

FIG. 2 is a rear side elevational view thereof;

FIG. 3 is a front side elevational view thereof; and,

FIG. 4 is a top plan view thereof.

The bottom of the portion of a chair back is plain and unornamented and forms no part of the claimed design. The broken lines, including those showing the remaining lower portion of the chair back and other components of the chair, including the headrest, seat, controls, armrests and base, are for illustrative purposes only and form no part of the claimed design. 

The ornamental design for a portion of a chair back, as shown and described. 